This invention relates to a method and a device for controlling a toner concentration of a dry developing agent used in an electrophotographic apparatus wherein an electrostatic latent image is developed by a two-component dry developing agent including toner and magnetizable carrier.
In such an apparatus, it is required to maintain the toner concentration of the developing agent, which is contained in a developing device thereof and serves to develop the electrostatic latent image, within a range capable of producing an optimum copy image. For this purpose, there have hitherto been known various methods, such as ones (1) for controlling the toner concentration of the developing agent by supplying a determined amount of toner to the developing agent stored in the developing device at determined intervals of the copy cycles, (2) for controlling the toner concentration of the developing agent by causing the passage of the developing agent through a funnel-shaped member having an electrical winding to detect changes in the permeability of the developing agent and varying the rate of supply of the toner to the developing agent in response to the changes in the permeability, and (3) for controlling the toner concentration of the developing agent by providing a restricted passage in the developing device for by-passing the developing agent, detecting the volume of the agent flowing through the passage and varying the rate of supply of the toner to the developing agent in response to changes in the volume of the developing agent. However, the method (1) is disadvantageous in that when a large number of copies is successively made by using an original having a relatively large black area, for example, a large amount of toner of the developing agent will be spent to make the toner concentration lowered below an appropriate value thereby degrading the image quality. In the methods (2) and (3), it is also disadvantageous that the devices are complicated.